01 - Place sliced red onions into a clean 2-cup heatproof jar. Add sliced garlic, red pepper flakes, and peppercorns on top.
02 - Combine apple cider vinegar, water, hot honey, granulated sugar, and kosher salt in a small saucepan. Heat over medium heat, stirring occasionally until sugar and salt completely dissolve. Bring to a gentle simmer.
03 - Carefully pour the hot brine over the onions in the jar. Ensure all onions are completely submerged in liquid. Press down with a spoon if necessary to fully cover onions.
04 - Allow jar to cool to room temperature uncovered. Cover tightly with lid and refrigerate for at least 1 hour before serving. For optimal flavor development, marinate overnight.
05 - Keep refrigerated in a sealed container. Properly stored, pickled onions will maintain quality for up to 2 weeks.